High capacity fiber transmission already provides the technical means to move very large amounts of data from node to node at reasonable cost. Broadband access systems are based on systems such as fiber to the home, fiber to the curb, coaxial cable, or wireless, and will serve to connect subscribers to local access nodes. As the number of asynchronous transfer mode (ATM) broadband subscribers grows, and total network traffic volume increases, the construction of an efficient access and tandem network will require very large ATM switches with aggregate capacities in the 100's to 1000's of gigabits per second (Gbit/s).
Innovations in network architecture may lead to a more distributed network of a larger number of smaller nodes, but the geographic clustering of traffic in cities and communities, the shrinking cost of collecting a large bandwidth over ring, tree, or star access networks, and the operational economics of fewer, larger sites is likely to continue to favor the concentration of traffic into exchanges serving 10,000 to 100,000 subscribers. Similarly, fewer but larger tandem switches will be more economical. Local and tandem exchanges capable of switching 5,000 to 80,000 Erlang of voice traffic are already common today. Extending from this existing telephone network capacity and assuming that traffic patterns and communities of interest do not change substantially, a simple estimate of switch sizes would be 0.4 to 5 Gbit/s at 64 kbit/s.
Traffic demand and average bit rates of terminals in the broadband network are less predictable. Average peak hour demand may range from a few 100 kbit/s to 10 Mbit/s or more per subscriber, depending upon the offered service. For example, digital video-on-demand, using MPEG2, could easily generate a network demand of 5 or 10 Mbit/s per household (the bottleneck in this scenario appears to be the video server capacity).
It would require a gross switching capacity of one Terabit/s to handle the aggregate demand of a 100,000 subscriber head end switch. Similarly, millions of already existing home computers could transfer data over a broadband network at peak rates well in excess of 10 Mbit/s, if such a network were offered economically.
Today, ATM switches which address both the data and the evolving multi-media market are being offered. These switches have capacities ranging from less than 600 Mbit/s to a few 10's of Gbit/s. The task of switching much larger amounts of point to multi-point or point-to-point traffic efficiently will have to be solved in future.
In U.S. Pat. No. 5,126,999, issued Jun. 30, 1992 (Munter et al), an ATM switch is described in which output segregated input buffers are operated on real-time by crosspoint selection circuits implementing a combined buffer fill/age algorithm.
In U.S. Pat. No. 5,241,536, issued Aug. 31, 1993 (Grimble et al), a timeslot utilization means is provided in an ATM switch for scheduling the earliest possible connection between an input port and output ports.
In U.S. Pat. No. 5,130,975, issued Jul. 14, 1992 (Akata), a timeslot scheduling unit in an ATM switch prevents the packets from collision in a space division switching unit. Each packet buffer unit at each port writes packets sequentially but reads out randomly in the timeslots assigned by the timeslot scheduling unit so that the throughput of the space division switching unit is improved.
In U.S. Pat. No. 5,157,654, issued Oct. 20, 1992 (Cisneros), a contention resolution technique for a large ATM switch is described. It utilizes cell address look-ahead in conjunction with parallel planes of self-routing cross-points, staggered time phased contention resolution and shared memory based input and output modules.
In U.S. Pat. No. 4,956,839, issued Sep. 11, 1990 (Torii et al), an ATM switch includes ATM line terminating units and a self-routing space switch.
In the applicant's co-pending application Ser. No. 08/352,405 filed on Dec. 8, 1994, the disclosure of which is hereby incorporated herein by reference, a large capacity ATM switch is described. The switch is based loosely on a space switch crosspoint, input and output buffers and substantially high speed links connecting them. A connection is set up between buffers by an exchange of connection control signals and trains of ATM cells are transmitted in bursts. A combination of high speed links and transmission of cells in bursts achieves high capacity switching with relatively low speed control signals.
It is desirable that a switch not only possess a large capacity but can also grow smoothly in capacity to its maximum, for example over a range from perhaps 50 up to 1000 Gbs and more. Such smooth expandability is the main stumbling block for practically all possible architectures. In terms of product and market requirements, the cost-per-port curve should be flat over the range expected to be used widely. In terms of physical realities, a switch inherently has a non-linear growth curve, a combination of a constant cost-per-port of the external I/O components, a logarithmic (theoretical best case) to square law characteristic for the switch core function, and a potentially square or higher law property in the switch control function.
In an article entitled "A Scalable ATM Switching System Architecture" by Fischer et al in IEEE Journal on Selected Areas in Communications Vol. 9, No. 8, Oct. 1991 pp 1299-1307, an ATM switching module is described. The article further discusses schemes using the modules to expand capacity in terms of the number of ports without service interruption.
The architecture according to the present invention employs a novel modular construction which makes use of high speed buses and buffers. The inventive modular construction take advantage of a burst switching mechanism similar to that described in applicant's above-referenced co-pending application.